Chemical Synthesis of Quillaic Acid, the Aglycone of QS-21

With the easily available protoescigenin as a starting material, a protocol to chemically synthesize quillaic acid was established. Benefiting from the scalability of each step comprising the novel protocol, the route featured robustness. Moreover, the devised protocol also exhibited outstanding flexibility, and other scarce and difficult-to-access oleanane-type aglycones, such as echinocystic acid, acacic acid lactone, as well as analogues derived from the D/E-ring rearrangement, C16,21 intramolecular etherification, and E-ring modification with C-substituents, could be conveniently accessed by sharing the common intermediates with quillaic acid.